Attenuators



W. R. JONES ATTENUATORS Nov. 20, 1962 Filed June 29. 1960 2 Sheets-Sheet1 hi U.

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S M R m m N R W m W u l W a H m l I O a 0 0 2 MW 0 o o o O 0 M c A o p wO 0 0 o m o o a 00 Oman n 0 oowo M o 0 BY TORNEY W. R. JONES Nov. 20,1962 ATTENUATORS Filed June 29. 1960 2 Sheets-Sheet FIG. 2

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United States Patent 3,065,435 ATTENUATORS William R. Jones, Hillsdale,N.J., assignor to International Business Machines Corporation, New York,N.Y., a corporation of New York Filed June 29, 1960, Ser. No. 39,628 7Claims. (1. 33381) This invention relates to attenuators and moreparticularly to variable attenuators which may be used to vary the powerlevel of the electromagnetic field in a guided wave system employingstrip transmission line, e.g., of the type disclosed in U.S. Patent No.2,913,686, granted November 17, 1959, which includes two spaced apartparallel ground plates, a center conductor disposed midway between thetwo ground plates and a dielectric material disposed between the centerconductor and each of the ground plates.

Satisfactory attenuators are available for use in many types of guidedwave transmission lines, e.g., in rectangular, circular and coaxial waveguides employed to transmit high frequencies, particularly in themicrowave region of the frequency spectrum. Elements such ascarboncoated cardshave been used as attenuators and even as variableattenuators in strip line systems, e.g., as described in SandersAssociates Handbook of Tri-Plate Microwave Components, 1956, in pages 53to 57. However, a satisfactory strip line attenuator has not beendeveloped which issuitable for use in all types of presently known striplines e.g., strip lines having single or double layer center conductors.One of the main problems in developing a satisfactory strip lineattenuator is that encountered in preserving the electrical balance ofthe transmission system. Prior art strip line attenuators useful in onetype of strip line system cannot be readily employed in a second type ofstrip line system due to electrical unbalance or mechanical fabricationproblems that are ecountered. Another disadvantage of prior art variableattenuators is that the resistive or lossy material of the attenuatingdevice has been inserted in the fringe of the electric field of thesystem, therefore, large values of attenuation must be accompanied bythe insertion of a large amount of resistive material.

An object of this invention is to provide an improved attenuator.

Another object of this invention is to provide an improved variableattenuator which may be employed in strip transmission lines.

. A further object of this invention is to provide an improved variableattenuator which may be used in all types of known strip transmissionlines.

' Yet another object of this invention is to provide an improvedvariable attenuator having its lossy or resistive material disposed inthe region of the maximum electric field of the strip line system.

Still another object of this invention is to provide an improvedattenuator in a strip transmission line which will not produce anelectrical unbalance in the line.

Still a further object of this invention is to provide an improved stripline attenuator which will provide a larger amount of attenuation with agiven amount of resistive material than can be produced by prior artattenuators.

In accordance with the present invention an attenuator for a strip linesystem has been provided which includes a center conductor having asection that is bifurcated so as to form two branches which are flaredto a distance at which there is no electrical coupling between thebranches and a thin lossy sheet disposed between the two branchesperpendicularly to a plane passing through each of the two branches ofthe center conductor.

An important advantage of the attenuator of the present invention isthat it will provide a large amount of at- Patented Nov. 20, 1962 icetenuation with small amounts of resistive material insertion withoutcreating an electrical unbalance in a strip transmission system.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

In the drawings:

FIG. 1 is a perspective view of a portion of a strip transmission linein which the attenuator of the present invention is located.

FIG. 2 is a plan view of a strip transmission line similar to thatillustrated in FIG. 1, but with the upper plate and upper dielectriclayer removed, showing two variable attenuators in a bifurcated sectionof the center conductor,

FIG. 3 is similar to FIG. 2 except that it shows the two attenuatorspositioned to provide minimum or zero attenuation,

FIG. 4 is a longitudinal cross sectional view of the strip line shown inFIG. 3 taken along line 44 which bisects the center conductor of thetransmission line,

FIG. 5 is a transverse cross sectional view of the strip lineillustrated in FIG. 4 taken along the line 55, and

FIG. 6 is an illustration of the magnitude of the electric fieldcomponent perpendicular to the center conductor measured along thecenter line of the center conductor.

Referring to the drawings in more detail, wherein like parts in thevarious figures are indicated by the same reference numeral, in FIG. 1there is shown a portion of a strip transmission line which includes alower electrically conductive plate 10 and an upper electricallyconductive plate 12 which is spaced from and arranged parallel to thelower plate 10. A conductive flat center conductor 14 having a widthsubstantially less than that of the plates 10 and 12, is disposed midwaybetween the plates 10 and 12. The center conductor 14 is held inposition between the lower and upper plates 10 and 12 by a lower layer16 of dielectric material and an upper layer 18 of dielectric materialdisposed between the center conductor 14 and the lower and upper plates10 and 12. The lower and upper dielectric layers 16 and 18 arepreferably made of a solid dielectric foam as described in a commonlyassigned application having Serial No. 824,003, filed June 30, 1959, byGeorge F. Bland, now abandoned, and in its divisional application havingSerial No. 96,968, filed March 20, 1961. The center conductor 14 may beconveniently made by utilizing well-known printed circuit techniques.The center conductor 14 has a section 20 bifurcated so as to form twobranches 22 and 24 which are flared for a distance and then form twoparallel lines 26 and 28. Each of the two branches 22 and 24 and each ofthe two parallel lines 26 and 28 have dimensions such that the even TEMmode impedance of one branch or line in the presence of the other isequal to 22 where Z is the impedance of the line or conductor which hasbeen bifurcated. A thin sheet 30, made of material having a lowdielectric constant, coated with a lossy material, such as carbonparticles, e.g., a metallized mica sheet, is disposed in a groove orslot 32 extending from the top of the upper plate 12 to the lower plate10 perpendicularly to the plates 10 and 12, and extending longitudinallyof the transmission line from a point within and midway between the twoparallel lines 26 and 28 to a point on the center line of the centerconductor 14 out side the bifurcated section 20 at a distance at leastequal to the length of the dielectric sheet 30. The dimension of thedielectric sheet 30 in the direction perpendicular to the plates 10 and12 is equal to the distance between the lower and upper plates 10 and 12and the sheet 30 should be sufficiently thin, e.g., .002 inch, so as topresent only a small discontinuity when in slot 32. If desired,

the dielectric sheet 30 or an arm 33 made of low dielectric materialconnected thereto, may extend out of the upper plate 12, provided theslot in the upper plate is very thin, so that the position of the sheet30 in the slot 32 may be conveniently controlled.

1 FIG. 2 is a plan view of a strip transmission line, with the upperplate and dielectric layer removed showing two lossy sheets, the sheet30 of FIG. 1 and a second sheet 30' similar to sheet 30, in thebifurcated section 20 of the center conductor 14. The sheets 30 and 30are arranged in the transmission line in the manner describedhereinabove in connection with FIG. 1. Fig. 2 shows the bifurcatedsection 20 having the flared branches 22 and 24 of FIG. 1 at one end ofthe section 20 and a second pair of flared branches 22 and 24' at theopposite end of the section 20 interconnected by the two parallel lines26 and 28. Each of the branches 22, 24, 22 and 24 and the parallel lines26 and 28 in the bifurcated section 2% have dimensions such that theeven mode impedance of one branch or line in the presence of the otheris equal to 2Z where Z is the impedance of the line which has beenbifurcated. Thus, each of the two transmission paths in the bifurcatedsection 20 has an impedance of 2Z so as to properly match the impedanceZ of the main line of the central conductor 14 connected to each end ofthe bifurcated section 20 at the vertices of the flared branches. Thelossy sheets 30 and 30' are disposed in slots 32 and 32' between thefirst pair of flared branches 22 and 24 and the second pair of flaredbranches 22' and 24, respectively, so as to pass through the vertices ofthe flared branches. A rack and pinion arrangement 34 has been providedso as to simultaneously adjust the position of the lossy sheets 30 and30' in the grooves 32 and 32', respectively. The rack and pinionarrangement 34 includes first and second racks 36 and 36, a pinion 33and a knurled knob 40 coupled to the pinion 38. The lossy sheet 30 isconnected to the first rack 36 by any suitable mechanical couplingelement 42 made of material with a low dielectric constant and, ifdesired, may be the same material used in the dielectric sheets 30 and30". The coupling element 42 should be sufficiently rigid so as to beable to push the lossy sheet 30 through the groove 32. The couplingelement 42 may pass through the slot or groove 32 and an extension 44thereof to the rack 36, in which case the coupling element 42 should besomewhat flexible, or, alternatively, the coupling element 42 may be aslender rod disposed between the lower and upper layers 16 and 18 of thetransmission line dielectric material. The lossy sheet 30 is coupled tothe second rack 36' in a manner similar to that disclosed hereinabove inconnection with the coupling of the lossy sheet 30 to the first rack 36.

FIG. 3 is similar to FIG. 2 but differs therefrom in that it shows thefirst and second racks 36 and 36 positioned so as to place the lossysheets 30 and 30' at locations spaced from the center conductor-so as toprovide minimum or zero attenuation.

FIG. 4 is a longitudinal cross sectional view of the strip lineillustrated in FIG. 3, taken along line 4-4, with the upper plate 12 andupper dielectric layer 18 included. This view clearly shows the lossysheets 30 and 30' extending between the lower and upper plates and 12and :also the position of the knurled knob 40 of the rack and pinionarrangement 34 on the upper plate 12.

FIG. 5 is a transverse cross sectional view of the strip transmissionline illustrated in FIG. 4 taken through the line 55. FIG. 5 clearlyshows the perpendicular relationship between the lossy sheet 30 and thecenter conductor 14. This figure also clearly shows that the lossy sheet30' passes through a line which bisects the center conductor 14 and thatequal amounts of the lossy sheet 30 are disposed above and below thecenter conductor 14-.

When a voltage is applied to the strip transmission line illustrated inthe above-mentioned figures, a component of the electric fieldperpendicular to the flat centerconductor along the center line of thecenter conductor, i.e.,

along line 4-4 of FIG. 3, has a magnitude as indicated in FIG. 6 of thedrawing. The arrows shown in FIG. 6 indicate the magnitude of theelectric field in the plane passing through the center line of centerconductor 14, i.e., line 44 of FIG. 3, perpendicularly to the plates 10and 12, the location of the plates 10 and 12 in that plane beingindicated by lines 10' and 12', respectively, and the line midwaybetween the two plates 10 and 12 in that plane being indicated by theline 4. It can be seen that the electric field in the slots 32 and 32outside of the bifurcated section 29 is at a maximum value but that itdecreases at increasing distances from the vertices of the branch lines22 and 24 and 22 and 24', since the region of high coupling is near thevertex of the branches 22 and 24 and the vertex of branches 22 and 24and the coupling decreases as the distance between the two branches 22and 24 increases and as the distance between the two branches 22 and 24increases until zero coupling is attained. The lines 26 and 28interconnecting the branches 22 and 24 and branches 22 and 24 arelocated outside of the coupling region and, therefore, although shownparallel, need not be positioned with respect to each other in anyparticular manner but they should have equal lengths. It should also beunderstood that if each of the resistive or lossy sheets 30 and 30' isnot disposed in the transmission line symmetrically with respect to thecenter conductor 14 the transmission line will become unbalanced andthus undesirable radiations will be produced. Thus, when the lossysheets 30 and 30' in the slot 32 are spaced the farthest distance apartfrom each other, as shown in FIG. 2 they are positioned in the portionor" the slot 32 which extends into the main line of the centralconductor 14 outside of the flared branches 22, 24, 22 and 24 so as tobe located within the strip transmission lines maximum electric fieldand when the lossy Sheets 36 and 30' are positioned as shown in FIGS. 3and 4 they are located in a minimum or zero electric field. When thelossy sheets 30 and 30" occupy positions in the slots 32 and 32,respectively, between the positions illustrated in FIGS. 2 and 3 or 4the sheets are located in electric fields of intermediate strength.Since it is well known that the attenuation in a transmission linedepends upon strength of the electric field through a' lossy material itcan be readily understood that the present invention provides a variableattenuator yielding attenuation values ranging from zero to maximum bymoving the lossy sheets 3% and 30, e.g., by turning knurled knob 40,only a short distance in the grooves 32 and 32', respectively. Ifdesired, the knurled knob 40 maybe associated with a convenient scalemounted on the upper plate 12.

The strip line variable attenuators illustrated in FIGS. 2, 3 and 4 maybe considered comprising a power divider at one end of the bifurcatedsection, e.g., flared branches 22 and 24, a power adder at the oppositeend of the bifurcated section, e.g., flared branches 22' and 24', if theamplitude and phase of the voltages in the two branches are equal, and apair of completely uncoupled parallel strip transmission lines, e.g.,lines 26 and 28, coupling the power divider to the power adder. Bygradually flaring the branches of the power divider and the power adder,the power division and addition is obtained without appreciablerefiection in the transmission line. Whenever a power divider isrequired in a system, a variable attenuator as illustrated in FIG. 1 ofthe drawing may conveniently be employed, which attenuator does notnecessitate the use of a power adder. When a power adder, as definedhereinabove is required in a system a variable attenuator as illustratedin FIG. 1 of the drawing also may conveniently be employed, whichattenuator .does not necessitate the use of a power divider.

Accordingly, an improved variable attenuator having a wide range ofattenuation values in a strip transmission line has been provided inaccordance with the present invention which has its lossy materialcompletely within the maximum electric field for maximum attenuation orpower loss and completely out of the electric field to provide a zero orminimum attenuation or loss.

Although the lossy sheets have been described as adjustable with respectto the center conductor 14, it should be understood that the sheets maybe fixed in one position to provide a fixed attenuator.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in forms and details maybe made therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. An attenuator comprising a pair of conductive plates spaced from eachother in a parallel relationship, a center conductor disposed betweensaid plates so as to be parallel thereto, said center conductor having asection bifurcated so as to form two flared branches, a lossy sheetdisposed within the two flared branches perpendicularly to a planepassing through each of said branches and means for moving said lossysheet between said flared branches along the longitudinal axis of saidcenter conductor.

2. An attenuator comprising a pair of conductive plates spaced from eachother in a parallel relationship, a center conductor disposed midwaybetween said plates so as to be parallel thereto, said center conductorhaving a section bifurcated so as to form two flared branches, a lossysheet disposed between the two flared branches perpendicularly to aplane passing through each of said branches and extending an equaldistance from said center conductor to each of said plates and means foradjusting the position of said lossy sheet between said flared branchesalong the longitudinal axis of said center conductor.

3. A variable attenuator comprising a pair of conductive plates spacedfrom each other in a parallel relationship, a center conductor disposedmidway between said plates so as to be parallel thereto, said centerconductor having a section bifurcated so as to form two flared branches,a lossy sheet having equal areas disposed between said center conductorand each of said plates and means for positioning said lossy sheetperpendicularly to said plates at a plurality of locations between saidflared branches along a line which bisects the angle formed by the twoflared branches.

4. A variable attenuator comprising a pair of conductive plates spacedfrom each other in a parallel relationship, a center conductor forcarrying electrical energy disposed midway between said plates so as tobe parallel thereto, said center conductor having a section bifurcatedso as to form two flared branches each having an area separated fromeach other by a distance at which the areas are electrically uncoupledfrom each other, a lossy sheet having equal areas disposed between saidcenter conductor and each of said plates and means for positioning saidlossy sheet perpendicularly to said plates at a plurality of positionsalong a line extending from a point midway between said two electricallyuncoupled areas to a point on the center line of said center conductorat a given distance outside of said bicurcated section.

5. A variable attenuator for a strip transmission line comprising a pairof conductive plates spaced from each other in a parallel relationship,a power divider having a pair of flared branches, a power adder having apair of flared branches and coupled to said power divider, means made ofa dielectric material for supporting said power divider and said poweradder midway between said plates, first and second lossy sheets disposedbetween said power divider and said plates and said power adder and saidplates, respectively, and means for positioning said first and secondlossy sheets perpendicularly to said plates at a plurality of locationswithin said power divider and said power adder, respectively, along theline bisecting the angle formed by said divider branches and along theline bisecting the angle formed by said adder branches, respectively.

6. A variable attenuator as set forth in claim 5 wherein saidpositioning means includes means for simultaneously adjusting theposition of said first and second lossy sheets with respect to saidpower divider and said power adder, respectively. I

7. An attenuator comprising a pair of conductive plates spaced from eachother in parallel relationship, a center conductor disposed midwaybetween said plates so as to be parallel thereto, said center conductorhaving a bifurcated section forming two branches, said branches beingflared to a given distance so as to form a substantially field freeregion therebetween, a thin lossy sheet disposed between said twobranches perpendicularly to a plane passing through each of the twobranches of said center conductor and means for adjusting said lossysheet at a plurality of positions throughout said bifurcated sectionalong a line which bisects the angle formed by the two flared branches.

References Cited in the file of this patent UNITED STATES PATENTS2,515,228 Hupcey July 18, b 2,619,538 Grant Nov. 25, 1952 2,807,785Wilds Sept. 24, 1957 2,913,686 Fubini Nov. 17, 1959 OTHER REFERENCESBarrett, Microwave Printed Circuits, volume MIT-3, No. 2, March1955,-pages 1-9, I.R.E. Transactions on Microwave Theory and Techniques.

